Polyolefins such as polyethylene, polypropylene, ethylene/α-olefin copolymers and propylene/α-olefin copolymers are used in various uses. For example, films formed of a straight-chain low-density polyethylene (LLDPE) are superior in various characteristics, for example, these films have excellent heat sealing characteristics, high softness and high toughness, are superior in water resistance, humidity resistance and chemical resistance and are inexpensive, and are therefore widely utilized from of old.
These polyolefins are typically produced by a liquid phase polymerization method such as solution polymerization and slurry polymerization. Particularly many LLDPEs are produced by copolymerizing ethylene with an α-olefin having 3 or more carbon atoms in the presence of a Ziegler catalyst by using a liquid phase polymerization method.
In the meantime, if such a polyolefin is produced by a vapor phase polymerization method, the polymer can be obtained in the form of a particle after polymerization and a step of precipitating particles and a step of separating particles from the polymerization solution become needless and therefore the production process can be simplified. This is the reason why studies concerning the production of polyolefins, particularly, LLDPEs by using a vapor phase polymerization method have been made energetically in recent years.
In the vapor phase polymerization method, polymerization is carried out with fluidizing solid particles consisting of a catalyst and a produced polyolefin by supplying polymerizing monomer gas from the lower part of a reactor to form a fluidized bed and to thereby run a polymerization reaction and withdrawing the produced polymer continuously or intermittently from the reactor.
In such a vapor phase polymerization method, there is a large problem concerning the removal of polymerization heat. Conventionally, a method is known in which a liquefiable saturated aliphatic hydrocarbon is supplied to a fluidized-bed reactor and the gas discharged from the reactor is compressed and cooled to liquefy a part of the saturated aliphatic hydrocarbon, to circulate the saturated aliphatic hydrocarbon to the reactor in a vapor-liquid mixed state, thereby removing polymerization heat.
However, even if the conventional heat removing method as aforementioned is adopted, it is difficult to remove heat evenly from the fluidized bed and therefore local heating in the fluidized bed is easily caused, giving rise to the problem that phenomena such as sheeting and melting tend to occur. When such a sheet or chunk polymer is produced, the polymer is deposited on a gas distributing plate disposed on the lower portion of the fluidized bed and the deposited polymer becomes an obstacle to remain a normal fluidizing state and also clogs the holes of the gas distributing plate, which occasionally hinders the normal operation of the reaction system significantly.
In the aforementioned vapor phase polymerization method, olefins are (co) polymerized in the presence of a solid catalyst, for example, such as a solid titanium type catalyst and support carrying type metallocene type catalyst to produce a polyolefin. Many of the foregoing solid catalysts have low fluidity, making it difficult to supply the catalyst to a polymerizing unit occasionally. Also, these catalysts are easily charged with electricity, leading to, for example, coagulation of the catalyst and adhesion of the catalyst to the wall surface of a polymerizing unit, affording opportunity for formation of a sheet or chunk polymer. Such a problem is improved to some extent by carrying a surfactant on the solid catalyst. However, this method has the problem of reduced polymerization activity.
For example, a method of improving the fluidity of a polymer powder is described in the publication of Japanese Patent Application Laid-Open No. 2000-313717, the method being characterized in that an aliphatic amide is supplied such that it exists in an amount of 0.1 to 400 ppm based on the weight of a polymerization powder in a powder fluidized circumstance where the content of water in the total gas is 2 ppm or less in a polymerization step or a step of storing the resulting polymer powder. In this publication, it is disclosed that static electricity of a polymer is prevented and the clogging caused by the generation of the sheet or chunk polymer as aforementioned is suppressed by using an aliphatic amide.
However, aliphatic amides poison a catalyst and drops the bulk density of the produced polyolefin by nature. Therefore, the use of a large amount of these aliphatic amides decreases catalyst activity and drops the bulk density of a polyolefin particle, giving rise to the problem of a remarkable reduction in the production efficiency of a polyolefin. Such a problem is known to be solved by using a polyoxyalkylene glycol (e.g., a polyalkylene oxide block) as shown in the publication of Japanese Patent Application Laid-Open No. 2000-313716 and in Japanese Patent Application Laid-Open No. 2000-327707. Specifically, it is disclosed that if polymerization is carried out in the presence of this compound, the amount of a chunk and sheet polymer to be produced is limited to the same level as in the case of using a higher aliphatic amide without any drop in the bulk density of the particle and high catalyst activity can be maintained.
However, the problem in the case of using a polyoxyalkylene glycol singly is that when changing reaction conditions significantly or when reaction conditions are disturbed, the fluidity of solid particles becomes ununiform temporarily. Even if the amount of a polyoxyalkylene glycol to be added is regulated, there is the case where the effect of suppressing the generation of a chunk or sheet polymer can be obtained insufficiently.
The inventors of the present invention have made earnest studies to solve the aforementioned problem and as a result, found that a polyolefin exhibiting high catalyst activity and a good particle quality (bulk density) can be produced while suppressing the generation of a chunk or sheet polymer even in the case where reaction conditions are disturbed by polymerizing an α-olefin in a vapor phase under the presence of at least one compound selected from (B) an aliphatic amide and (C) a nonionic surfactant constituted only of carbon, oxygen and hydrogen atoms and (A) a saturated aliphatic hydrocarbon. The present invention was thus completed. It has been also found that, particularly, in the case of carrying out vapor phase polymerization in the presence of the saturated aliphatic hydrocarbon (A) with allowing the aliphatic amide (B) and the nonionic surfactant (C) constituted only of carbon, oxygen and hydrogen atoms to coexist, the amount of a chunk or sheet polymer to be produced is significantly restricted even in the case where a sudden variation in reaction conditions is caused and the reaction system is disturbed and also the catalyst activity is maintained for a long period of time.